1. Field of the Invention
The present invention relates to an optical measuring apparatus which enables a coordinate positioning machine (such as a machine tool) to determine the position of an object relative to a reference point. It may, for example, be employed on a machine tool for toolsetting operations.
2. Description of Related Art
A known tool setting device for use on a machine tool includes a light source which generates a fine beam of light which is incident upon a detector. During a toolsetting operation, the machine is operated to move the tool in a direction transverse to the direction of propagation of the light beam until a part of the tool interrupts passage of the light beam. Detection of this interruption is used to produce a trigger signal in the detecting unit, which is used by the machine to establish the relative position of its moving parts in order to determine dimensions of the tool. Such devices are known, for example, from German Patent Nos. DE 42 385 04 and DE 42 448 69, French Patent No. 2,343,555, European Patent No. 98,930 and U.S. Pat. No. 4,518,257. The devices may be used additionally for measuring the length or diameter of a tool to monitor tool breakage or wear.
The devices disclosed in the above-mentioned patent specifications use a narrow light beam into or through which the tool is passed. The detection units detect when the tool breaks into the beam from the resulting drop in the intensity of the light falling on them. The trigger signal may be produced as a result of a predetermined drop in the intensity of light falling on the detector as the tool enters the beam.
A problem which arises with such optical measuring apparatus is that coolant used on the machine can drip through the beam, or be thrown off the rotating tool into the beam, during the measuring operation and give rise to false trigger signals.
One method of overcoming this problem which is currently used, is to program the software in the machine controller to perform several measurements until a pre-selected number of measurements falling within a given tolerance have been obtained. The position of the tool is then assumed to be the average of these measurements. This method can give rise to an unacceptable increase in the measurement cycle time if a significant number of repeat measurements have to be taken.
The present invention seeks to alleviate this problem by providing a method of measurement which can differentiate between a genuine tool detection signal, and a signal produced by a coolant drip.
According to one aspect of the present invention there is provided a method of making measurements of an object on a machine tool using an optical measuring apparatus which includes a light source which generates a beam of light which is incident upon a detector, the method comprising the steps of:
generating a detection signal within the detector each time the beam is interrupted;
evaluating the frequency and/or duration of the occurrences of said detection signals;
emitting an output signal from the detector only if a further detection signal is also present within the detector in a specified time interval from the generation of an earlier detection signal.
The timing of the detection signals can be achieved in various ways.
In one embodiment of the invention the tool is rotated, preferably, at a known specific speed. This gives rise to the generation of a regular sequence of said signals within the detector as the cutting edge (or edges) of the tool interrupt the beam. The generation of the first one of the said signals is used to initiate a timing sequence within the detector which sets a time interval (t1) substantially equal to the time taken for one revolution of the tool, followed by a second time interval (t2) which is substantially shorter than (t1). If it is the tool which has generated the signals in the detector then a second signal will be generated in the time interval (t2) as the cutting edge of the tool comes round again, and if this happens, the detector emits the output signal.
Alternatively, the generation of a detection signal within the detector can be used to start a clock which emits pulses of short duration synchronised with the speed of rotation of the tool. Again, if a second detection signal is generated within the detector during such a pulse then the detector emits an output signal. A number of clocks may be used which start sequentially as the detector generates its detection signals, and each of which stops if no second detection signal is generated within the detector during the next one of its pulses.
The invention also includes optical measuring apparatus for carrying out the method which comprises a light source for generating a light beam and a detector for receiving said beam and which generates a signal when the beam is interrupted, wherein the detector includes a detection circuit which generates a detection signal each time the beam is interrupted, and signal processing means for evaluating the frequency and/or duration of the occurrences of said detection signals and which emits an output signal only if a second signal is generated by the detection circuit within a specified time interval after the occurrence of an earlier detection signal.